In a fuel cell power systems for fuel cell vehicles, the electrical load may vary rapidly with time by as much as 0–100%, which is a major difference from fixed type fuel cell power plants. Although it is required to make the operating load of a fuel cell power system follow the electrical load in order that the fuel cell power system supplies electric power without any excess or deficiency, it is difficult to make the operating load of the fuel cell power system follow the electrical load. In JP-A-H4-51466 published by the Japanese Patent Office in 1996, a battery is connected to the output side of the fuel cell to compensate the delay of the fuel cell power system. When the electrical load exceeds the output of the fuel cell, the insufficiency is compensated by the battery, and when the electrical load is less than the output of the fuel cell, the excess power is stored in the battery.